Say what to do about the running operation, not just that it blocks

Review feedback: "Stop it first" is the only way out the old text offered,
and stopping is rarely what the user wants. Waiting for the operation to
finish works just as well, so the notification now names both.

src/slic3r/GUI/Plater.cpp

@@ -14974,7 +14974,8 @@ bool Plater::priv::undo_redo_blocked_by_job()
         return false;
     notification_manager->push_notification(NotificationType::CustomNotification,
                                             NotificationManager::NotificationLevel::RegularNotificationLevel,
-                                            _u8L("Cannot undo or redo while an operation is running. Stop it first."));
+                                            _u8L("Cannot undo or redo while an operation is running. Stop the operation, or wait "
+                                                 "for it to finish and then retry."));
     return true;
 }